linux-kernel - RE: [PATCH v4 12/21] kvm: x86: Intercept #NM for saving IA32_XFD

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Date:   Wed, 5 Jan 2022 00:27:09 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Zhong, Yang" <yang.zhong@...el.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "jing2.liu@...ux.intel.com" <jing2.liu@...ux.intel.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Zeng, Guang" <guang.zeng@...el.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>
Subject: RE: [PATCH v4 12/21] kvm: x86: Intercept #NM for saving IA32_XFD_ERR

> From: Sean Christopherson <seanjc@...gle.com>
> Sent: Wednesday, January 5, 2022 4:01 AM
> 
> On Wed, Dec 29, 2021, Yang Zhong wrote:
> > +static void handle_nm_fault_irqoff(struct kvm_vcpu *vcpu)
> > +{
> > +	rdmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
> > +	kvm_queue_exception(vcpu, NM_VECTOR);
> 
> This is still wrong, even though no additional supported is needed to support
> nested XFD.  If L1 wants to intercept #NM, then KVM must not inject the #NM
> and
> must not read XFD_ERR.

Sure. This v4 was sent out before we settled down this open in v3 
discussion. We did fix it later and posted the sample logic here:

https://lore.kernel.org/all/BN9PR11MB52767789D59239DF5DD524758C469@BN9PR11MB5276.namprd11.prod.outlook.com/

Will include this fix in v5.

Thanks
Kevin